In recent years, with the multimediatization of information in cellular mobile communication systems as represented by cell phones and so on, it is becoming popular to transmit not only audio data, but also great amounts of data such as still pictures, moving pictures and so on. To realize the transmission of such great amounts of data, a technology in which a high-frequency wireless band is used to obtain a high-transmission rate is being actively studied.
When high frequency radio bands are utilized, a high transmission rate can be expected in a short distance, but attenuation due to a transmission distance is significant. Therefore, when high frequency radio bands are utilized in a real system, for example, the coverage area of a base station becomes small, and it is necessary to provide more base stations. Providing base stations require significant costs, and therefore it is strongly desired to suppress an increase in the number of base stations and provide the above communication services.
To address these demands, a technology is investigated in which, a mobile station is used as a relay station, and a communication network is formed directly between a base station and a mobile station, while another communication network (ad-hoc network) is formed via the relay station, and the reception signals of both communication networks are combined at the base station to obtain diversity gain (see, for example, Patent Document 1).    Patent Document 1: Japanese Patent Application Laid-Open No. 2001-189971